Hafnium metallocene compound

ABSTRACT

A very efficient catalyst system for the polymerization of 1-olefins consists of an aluminoxane and a metallocene which is a compound of the formula I ##STR1## in which R 1  and R 2  denote hydrogen atoms, halogen atoms or hydrocarbon radicals and R 3  and R 4  are mononuclear or polynuclear hydrocarbon radicals which can form a sandwich structure with the central atom and which are linked to one another by a --R 6   m  --R 5  --R 7   n  -- bridge containing a hetero atom and consisting of one or several members. 
     The catalyst system produces polymers having a high molecular weight.

This is a divisional of Ser. No. 08/229,095 fled Apr. 18, 1994, which is a continuation of Ser. No. 07/842,796 filed Feb. 27, 1992, now abandoned which in turn is a continuation of Ser. No. 07/548,194 filed Jul. 5, 1990, now abandoned which again in turn is a continuation of Ser. No. 07/321,365 filed Mar. 9, 1989, now abandoned.

The invention relates to a process for the preparation of 1-olefin polymers having high isotacticity, narrow molecular weight distribution and high molecular weight.

Soluble metallocene catalysts based on bis(cyclopentadienyl)zirconium-alkyl or bis(cyclopentadienyl)zirconium halide in combination with oligomeric aluminoxanes are known from the literature. This system can be used to polymerize ethylene and propylene with moderate activity, however, no isotactic polypropylene is obtained.

Furthermore, the catalyst system bis(cyclopentadienyl)diphenyltitanium/methylaluminoxane is known to be capable of converting propylene into stereoblock polymers, that is, polypropylene having more or less long isotactic sequences (cf. U.S. Pat. No. 4,522,982). Distinct disadvantages of this catalyst system are the polymerization temperatures (0° C. to -60° C.) which are irrelevant for industry and the entirely unsatisfactory catalyst activities.

The preparation of isotactic polypropylene can be achieved by means of ethylenebis(4,5,6,7-tetrahydro-1-indenyl)zirconiumdichloride together with an aluminoxane in a suspension polymerization reaction (cf. EP-A 185,918). The polymer has a narrow molecular weight distribution, which is advantageous for certain applications, for example for high-performance injection molding.

At the same time, the catalyst system has a series of shortcomings. The polymerization is carried out in toluene, which has to be purified at great expense and freed from moisture and oxygen. Moreover, the bulk density of the polymer is too small and the grain morphology and particle size distribution unsatisfactory.

However, a serious disadvantage of this known process is that at the polymerization temperatures which are of interest in industry only polymers having unacceptably low molecular weights can be prepared.

A special preactivation method of the metallocene/aluminoxane system, which leads to a remarkable increase in the activity of the catalyst system and to a significant improvement in the grain morphology of the polymer has also been proposed (cf. DE 3,726,067). However, this does not significantly affect the value of the molecular weight.

Furthermore, catalysts based on ethylenebis(indenyl)hafnium dichloride and ethylenebis(4,5,6,7-tetrahydro-1-indenyl)hafnium dichloride and methylaluminoxane, by means of which higher-molecular-weight polypropylenes can be prepared by suspension polymerization, are known (cf. J. A. Ewen et al., J. Am. Chem. Soc. 109 (1987) 6544). However, under industrially interesting polymerization conditions, the value of the molecular weight of the polymers thus produced is still unsatisfactory.

The object was to find a catalyst which produces polymers having an even higher molecular weight.

It has been found that the use of certain hetero atom bridged hafnocene systems allows a significant increase in the molecular weight.

Accordingly, the invention relates to a process for the preparation of a 1-olefin polymer by polymerization of a 1-olefin of the formula R--CH═CH₂ in which R is an alkyl group having 1 to 28 carbon atoms, or copolymerization of these olefins with one another or with ethylene, at a temperature from -60° to 200° C., at a pressure from 0.5 to 60 bar, in solution, in suspension or in the gas phase, in the presence of a catalyst which consists of a metallocene as transition metal compound and an aluminoxane of the formula (II) ##STR2## for the linear type and/or of the formula (III) ##STR3## for the cyclic type, where in the formulae (II) and (III) R¹¹ denotes a C₁ -C₆ -alkyl group and p is an integer from 2 to 50, which comprises carrying out the polymerization in the presence of a catalyst whose transition metal component is a compound of the formula (I) ##STR4## in which R¹ and R² are identical or different and denote a hydrogen atom, a halogen atom, a C₂ -C₁₀ -alkyl group, a C₆ -C₁₀ -aryl group, a C₂ -C₁₀ -alkenyl group, a C₇ -C₄₀ -arylalkyl group, a C₇ -C₄₀ -alkylaryl group or a C₈ -C₄₀ -arylalkenyl group,

R₃ and R₄ are identical or different and denote a mononuclear or polynuclear hydrocarbon radical which can form a sandwich structure with the central atom,

R⁵ denotes ##STR5## ═BR⁸, ═AlR⁸, --Ge--, --Sn--, --O--, --S--, ═S═O, ═SO₂, ═NR⁸, ═PR⁸ or ═P(O)R⁸, in which

R⁸, R⁹ and R¹⁰ are identical or different and denote a hydrogen atom, a halogen atom, a C₁ -C₁₀₋ -alkyl group, a C₆ -C₁₀ -aryl group, a C₂ -C₁₀ -alkenyl group, a C₇ -C₄₀ -arylalkyl group, a C₈ -C₄₀ -arylalkenyl group or a C₇ -C₄₀ -alkylaryl group or

R⁸ and R⁹ or R⁸ and R¹⁰ each form a ring together with the atoms linking them,

M denotes silicon, germanium or tin and q denotes 1, 2 or 3,

R⁶ and R⁷ are identical or different and denote a ═CR⁸ R⁹ group, in which R⁸ and R⁹ have the abovementioned meaning, m and n are identical or different and denote zero, 1 or 2, where m+n is zero, 1 or 2,

and whose activator is also an aluminoxane of the formula (II) or (III).

Various transition metal compounds can be used for the process according to the invention. They are stereo-rigid chiral metallocenes of the formula (I) ##STR6## R¹ and R² are identical or different and denote a hydrogen atom, a C₁ -C₁₀ -alkyl group, preferably a C₁ -C₃ -alkyl group, a C₈ -C₁₀ -aryl group, preferably a C₆ -C₈ -aryl group, a C₂ -C₁₀ -alkenyl group, preferably a C₂ -C₄ -alkenyl group, a C₇ -C₄₀ -arylalkyl group, preferably a C₇ -C₁₀ -arylalkyl group, a C₇ -C₄₀ -alkylaryl group, preferably a C₇ -C₁₂ -alkylaryl group, a C₈ -C₄₀ -arylalkenyl group, preferably a C₈ -C₁₂ -arylalkenyl group or a halogen atom, preferably chlorine.

R³ and R⁴ are identical or different, preferably identical, and denote a mononuclear or polynuclear hydrocarbon radical which together with the central atom can form a sandwich structure. Examples of this type of radical are the indenyl, tetrahydroindenyl or cyclopentadienyl group and heteroaromatic ligands.

R⁵ is ##STR7## ═BR⁸, ═AlR⁸, --Ge--, --Sn--, --O--, --S--, ═S═O, ═SO₂, ═NR⁸, ═PR⁸ or ═P(O)R⁸, in which R⁸, R⁹ and R¹⁰ are identical or different and denote a hydrogen atom, a halogen atom, preferably fluorine, a C₁ -C₁₀ -alkyl group, preferably C₁ -C₄ -alkyl group, a C₆ -C₁₀ -aryl group, preferably a C₆ -C₈ -aryl group, a C₂ -C₁₀ -alkenyl group, preferably a C₂ -C₄ -alkenyl group, a C₇ -C₄₀ -arylalkyl group, preferably a C₇ -C₁₀ -arylalkyl group, a C₈ -C₄₀ -arylalkenyl group, preferably a C₈ -C₁₂ -arylalkenyl group or a C₇ -C₄₀ -alkylaryl group, preferably a C₇ -C₁₂ -alkylaryl group, or R⁸ and R⁹ or R⁸ and R¹⁰ each form a ring together with the atoms linking them.

M is Si, Ge or Sn and q is 1, 2 or 3,

R⁵ is preferably ═SiR⁸ R⁹, --S--, ═S═O or ═PR⁸,

R⁶ and R⁷ are identical or different and denote a ═CR⁸ R⁹ group, in which R⁸ and R⁹ have the abovementioned meaning,

m and n are identical or different and denote zero, 1 or 2, where m+n is zero, 1 or 2. Preferably, m and n are zero or 1.

The optically active metallocenes are used as racemate for the preparation of highly isotactic poly-1-olefins. However, it is also possible to use the pure R or S forms. An optically active polymer can be prepared by means of these pure stereoisomeric forms. However, it is necessary to separate off the meso form of the metallocenes, since the polymerization-active center (the metal atom) in these compounds is not longer chiral due to reflection symmetry on the central metal and can therefore not produce any highly isotactic polymers.

The principle of the separation of the stereoisomers is known.

The metallocenes described above can be prepared according to the following reaction scheme: ##STR8##

The metallocene compounds which are particularly preferably used are rac-bisindenyl(dialkylsilyl)hafnium dichlorides and rac-bisindenyl(arylalkylsilyl)hafnium dichlorides.

The activator is an aluminoxane of the formula (II) ##STR9## for the linear type and/or of the formula (III) ##STR10## for the cyclic type. In these formulae, R¹¹ denotes a C₁ -C₆ -alkyl group, preferably methyl, ethyl or isobutyl, in particular methyl, and p denotes an integer from 2 to 50, preferably 15 to 40.

The aluminoxane can be prepared, for example, by different methods.

In one of the processes, finely powdered copper sulfate pentahydrate is suspended in toluene, and trialkylaluminum is added in a glass flask under inert gas at about -20° C. in such an amount that about 1 mole of CuSO₄ ×5H₂ O is present for every four aluminum atoms. After slow hydrolysis with the elimination of alkane, the reaction mixture is left at room temperature for 24 to 48 hours, in the course of which it may have to be cooled to prevent the temperature from rising above 30° C. The aluminoxane dissolved in toluene is then freed from copper sulfate by filtration, and the solution is concentrated under a vacuum. It is assumed that in this preparation process the low-molecular-weight aluminoxanes are condensed to higher oligomers with the elimination of trialkylaluminum.

Aluminoxanes are also obtained by reacting trialkylaluminum, preferably trimethylaluminum, dissolved in an inert aliphatic or aromatic solvent, preferably heptane or toluene, at a temperature from -20° to 100° C. with hydrated aluminum salts, preferably aluminum sulfate. In this reaction, the ratio by volume between the solvent and the alkylaluminum used is 1:1 to 50:1--preferably 5:1--and the reaction time, which can be monitored by the elimination of the alkane, is 1 to 200 hours--preferably 10 to 40 hours.

Of the hydrated aluminum salts, in particular those are used which are highly hydrated. Particular preference is given to hydrated aluminum sulfate, in particular the compounds Al₂ (SO₄)₃ ×18H₂ O and Al₂ (SO₄)₃ ×16H₂ O which are particularly highly hydrated at 18 mole and 16 mole of H₂ O/mole of Al₂ (SO₄)₃.

A further variation for the preparation of aluminoxanes consists in dissolving trialkylaluminum, preferably trimethylaluminum, in the suspending agent which was initially introduced into the polymerization kettle, preferably in the liquid monomer, in heptane or toluene, and then reacting the aluminum compound with water.

Apart from the processes for the preparation of aluminoxanes described above, there are others which are useful.

It is preferred to preactivate the metallocene with an aluminoxane of the formula (II) and/or (III) before it is used in the polymerization reaction. This significantly increases the polymerization activity.

The preactivation of the transition metal compound is carried out in solution. Preferably, the metallocene is dissolved in a solution of the aluminoxane in an inert hydrocarbon. Suitable inert hydrocarbons are aliphatic or aromatic hydrocarbons.

Preferably, toluene is used.

The concentration of the aluminoxane in the solution is in the range from about 1% by weight to the saturation limit, preferably 5 to 30% by weight, in each case relative to the entire solution. The metallocene can be used in the same concentration, but preferably it is used in an amount of 10⁻⁴ -1 mole per mole of aluminoxane. The preactivation time is 5 minutes to 60 hours, preferably 5 to 60 minutes. The activation is carried out at a temperature from -78° C. to 100° C., preferably 0° to 70° C.

A considerably longer preactivation is possible, usually has no activity-increasing or activity-reducing effect, but can be quite appropriate for the purpose of storage.

Preactivation hardly increases the molecular weight of the polymer at all. Preferably, the same aluminoxane is used for the preactivation and the polymerization.

The catalyst to be used according to the invention is employed for the polymerization of 1-olefins of the formula R--CH═CH₂, in which R denotes an alkyl radical having 1 to 28 carbon atoms, preferably 1 to 10 carbon atoms, in particular one carbon atom, for example propylene, 1-butene, 1-hexene, 4-methyl-1-pentene and 1-octene. Propylene is particularly preferred. Furthermore, the catalyst is also used for the copolymerization of these olefins with one another or with ethylene, it being possible to incorporate in the product more than 50% by weight of ethylene by polymerization.

The polymerization is carried out in a known manner in solution, in suspension or in the gas phase, continuously or batchwise, in one or more steps at a temperature from -60° to 200° C., preferably -20 to 120, in particular 0° to 80° C. The pressure is 0.5 to 60 bar. Polymerization in the pressure range from 5 to 60 bar, which is of particular interest in industry, is preferred.

The metallocene compound is employed in a concentration, relative to the transition metal, of 10⁻³ and 10⁻⁷, preferably 10⁻⁴ to 10⁻⁶ mole of transition metal per dm³ of solvent or per dm³ of reactor volume. The aluminoxane is employed in a concentration of 10⁻⁴ to 10⁻¹ mole, preferably 10⁻³ to 10⁻² mole, per dm³ of solvent or per dm³ of reactor volume. Higher concentrations are, however, in principle also possible.

It is advantageous first to stir the aluminoxane for a few minutes together with the polymerization liquid phase before the addition of the metallocene. The stirring time is preferably 10 to 30 minutes. However, it is also possible to stir for a shorter period of time without any great losses and a longer stirring time has no significant effect on the result of polymerization.

The polymerization is carried out in an inert solvent customary for the Ziegler low-pressure process. For example in an aliphatic or cycloaliphatic hydrocarbon; such a solvent is, for example, butane, pentane, hexane, heptane, isooctane, cyclohexane, methylcyclohexane. A benzine or hydrogenated diesel oil fraction which has been carefully freed from oxygen, sulfur compounds and moisture can also be used. Toluene is also suitable. Preferably, the monomer to be polymerized is used as solvent or suspending agent. The molecular weight of the polymers can be regulated in a known manner; preferably, hydrogen is used for this. The duration of the polymerization can be of any desired length, since the catalyst system to be used according to the invention shows only a slight time-dependent decrease of the polymerization activity.

The polymers prepared by the process according to the invention have a high molecular weight, a very narrow molecular weight distribution and a high isotacticity.

While polymers prepared by means of the metallocenes based on zirconium show the known dependence of the molecular weight on the polymerization temperature (increase in polymerization temperature leads to decrease in molecular weight of the polymers), it has been found that the homologous hafnium systems at a higher polymerization temperature simultaneously also produce polymers having a higher molecular weight. If hetero atom-bridged Hf metallocenes are used, the molecular weight compared to metallocenes having a carbon bridge is increased even more. Furthermore, at a high polymerization temperature, the activity is simultaneously also significantly higher than at lower polymerization temperatures.

The examples which follow are intended to illustrate the invention. The symbols have the following meanings:

VN=viscosity number in cm³ /g,

M_(w) =average molecular weight in g/mol,

M_(w) /M_(n) =molecular weight distribution determined by gel permeation chromatography (GPC) and

II=isotactic index determined by ¹³ C-NMR spectroscopy

EXAMPLE 1

A dry 16 dm³ kettle was flushed with nitrogen, and 10 dm³ of liquid propylene were added.

70 cm³ of a methylaluminoxane solution in toluene (corresponding to 68 mmol of Al, average oligomerization degree n=30) were added, and the mixture was stirred at 30° C. for 15 minutes. At the same time, 50.8 mg (0.095 mmol) of rac-bisindenyl(dimethylsilyl)hafnium dichloride were dissolved in 35 cm³ of a methylaluminoxane solution in toluene (34 mmol of Al), and the solution was preactivated by being left to stand for 30 minutes. The solution was then added to the kettle.

The polymerization system was brought to a temperature of 70° C. and maintained at this temperature for 5 hours. 2.15 kg of polypropylene were obtained. The activity of the metallocene was 8.5 kg of PP/g of metallocene×h. VN=152 cm³ /g, M_(w) =168,400, M_(w) /M_(n) =2.4, II=94.3%, melting point 155° C.

EXAMPLE 2

A dry 16 dm³ kettle was flushed with nitrogen, and 10 dm³ of liquid propylene were added.

50 cm³ of a methylaluminoxane solution in toluene (corresponding to 68 mmol of A1, average oligomerization degree n=30) were added, and the mixture was stirred at 30° C. for 15 minutes. At the same time, 51.3 mg (0.096 mmol) of rac-bisindenyl(dimethylsilyl)hafnium dichloride were dissolved in 25 cm³ of a methylaluminoxane solution in toluene (34 mmol of Al), and the solution was preactivated by being left to stand for 15 minutes. The solution was then added to the kettle.

The polymerization system was brought to a temperature of 60° C. and maintained at this temperature for 5 hours. 1.08 kg of polypropylene were obtained. The activity of the metallocene was 4.2 kg of PP/g of metallocene×h. VN=105 cm³ /g, M_(w) =110,900, M_(w) /M_(n) =2.0, II=95.0%, melting point 145° C.

COMPARATIVE EXAMPLE A

The procedure of Example 2 was repeated, except that 24.9 mg (0.05 mmol) of rac-ethylenebisindenylhafnium dichloride were used and the polymerization time was 4 hours. 1.18 kg of polypropylene were obtained. The activity of the metallocene was therefore 11.8 kg of PP/g of metallocene×h. VN=65 cm³ /g, M_(w) =60,900, M_(w) /M_(n) =2.5, II=94.8%, melting point 135° C.

EXAMPLE 3

Analogously to Example 1, 25.2 mg (0.047 mmol) of rac-bisindenyl(dimethylsilyl)hafnium dichloride were used and the polymerization time was 10 hours. 2.31 kg of polypropylene were obtained. The activity of the metallocene was 9.2 kg of PP/g of metallocene×h. VN=149 cm³ /g, M_(w) =162,800, M_(w) /M_(n) =2.1, II=95.1%, melting point 156° C.

EXAMPLE 4

The procedure of Example 1 was repeated, except that the polymerization time was 2 hours. 1.09 kg of polypropylene were obtained. The activity of the metallocene was 10.7 kg of PP/g of metallocene×h. VN=167 cm³ /g, M_(w) =172,100, M_(w) /M_(n) =2.3, II=95.0%, melting point 156° C.

EXAMPLE 5

A dry 16 dm³ kettle was flushed with nitrogen, and 10 dm³ of liquid propylene were added.

50 cm³ of a methylaluminoxane solution in toluene (corresponding to 68 mmol of Al, average oligomerization degree n=23) were added, and the mixture was stirred at 30° C., for 15 minutes. At the same time, 25.7 mg (0.043 mmol) of rac-bisindenyl(phenyimethylsilyl)hafnium dichloride were dissolved in 25 cm³ of a methylaluminoxane solution in toluene (34mmol of Al), and the solution was preactivated by being left to stand for 15 minutes. The solution was then added to the kettle. The polymerization system was brought to a temperature of 70° C. and maintained at this temperature for 5 hours. 320 g of polypropylene were obtained. The activity of the metallocene was therefore 2.5 kg of PP/g of metallocene×h. VN=247 cm³ /g, M_(w) =288,400, M_(w) /M_(n) =2.6, II=96.4%.

EXAMPLE 6

The procedure of Example 5 was repeated, except that 25 mg (0.047 mmol) of rac-bisindenyl(dimethylsilyl)hafnium dichloride were used. 1.03 kg of polypropylene were obtained. The activity of the metallocene was 8.2 kg of PP/g of metallocene×h. VN=147 cm³ /g, M_(w) =161,625, M_(w) /M_(n) =2.5, II=94.8%, melting point 156° C.

EXAMPLE 7

The procedure of Example 5 was repeated, except that 71.0 mg (0.119 mmol) of rac-bisindenyl(phenylmethylsilyl)hafnium dichloride were used and the polymerization temperature was 60° C. 220 g of polypropylene were obtained. The activity of the metallocene was therefore 619 g of PP/g of metallocene×h. VN=181 cm³ /g, M_(w) =205,900, M_(w) /M_(n) =2.3, II=98.5%.

EXAMPLE 8

The procedure of Example 5 was repeated, except that 104.3 mg (0.175 mmol) of rac-bisindenyl(phenylmethylsilyl)hafnium dichloride were used, the polymerization temperature was 50° C. and the polymerization time was increased to 8 hours. 350 g of polypropylene were obtained. The activity of the metallocene was therefore 420 g of PP/g of metallocene×h. VN=177 cm³ /g, M_(w) =200,800, M_(w) /M_(n) =2,3, II=97.2%.

COMPARATIVE EXAMPLE B

The procedure of Example 8 was repeated, except that 49.9 mg (0.099 mmol) of rac-ethylenebisindenylhafnium dichloride were used and the polymerization temperature was 5 hours. 2.3 kg of polypropylene were obtained. The activity of the metallocene was therefore 9.2 kg of PP/g of metallocene×h. VN=57.1 cm³ /g, M_(w) =51,000, M_(w) /M_(n) =2.2, II=97.1%.

EXAMPLE 9

The procedure of Example 5 was repeated, except that 257 mg (0.43 mmol) of rac-bisindenyl(phenylmethylsilyl)hafnium dichloride were used and the polymerization temperature was 40° C. 200 g of polypropylene were obtained. The activity of the metallocene was 156 g of PP/g of metallocene×h. VN=147 cm³ /g, M_(w) =162,500, M_(w) /M_(n) =2.6, II=97.6%.

COMPARATIVE EXAMPLE C

The procedure of Example 9 was repeated, except that 144.1 mg (0.285 mmol) of rac-ethylenebisindenylhafnium dichloride were used and the polymerization time was 2 hours. 1.24 kg of polypropylene were obtained. The activity of the metallocene was therefore 4.3 kg of PP/g of metallocene×h. VN=60.2 cm³ /g, M_(w) =55,000, M_(w) /M_(n) =2.7, II=97%. 

We claim:
 1. A compound comprising a metallocene of the formula (I) ##STR11## in which R¹ and R² are identical or different and denote a hydrogen atom a halogen atom, a C₁ -C₁₀ -alkyl group, a C₆ C₁₀ -aryl group, a C₂ -C₁₀ -alkenyl group, a C₇ -C₄₀ -arylalkyl group, a C₇ -C₄₀ -alkylaryl group or a C₈ -C₄₀ -arylalkenyl group,R³ and R⁴ are identical or different and denote a mononuclear or polynuclear hydrocarbon radical which can form a sandwich structure with the central atom, R⁵ denotes ##STR12## in which R⁸ denotes a halogen atom, a C₆ -C₁₀ -aryl group, a C₇ -C₄₀ -arylalkyl group, a C₈ -C₄₀ -arylalkenyl group or a C₇ -C₄₀ -alkylaryl group, R⁹ and R¹⁰ are identical or different and denote a hydrogen atom, a halogen atom, a C₁ -C₁₀ -alkyl group, a C₆ -C₁₀ -aryl group, a C₂ -C₁₀ -alkenyl group, a C₇ -C₄₀ -arylalkyl group, a C₈ -C₄₀ -arylalkenyl group or a C₇ -C₄₀ -alkylaryl group or R⁸ and R⁹, or R⁸ and R¹⁰ each form a ring together with the atoms linking them, M denotes silicon or germanium and q denotes 1, 2 or 3, or R⁵ denotes ##STR13## ═BR⁸, ═AlR⁸, --Sn--, --O--, ═SO₂, ═NR⁸, ═RP⁸ or ═P(O)R⁸, in which R^(8'), R^(9') and R^(10') are identical or different and denote a hydrogen atom, a halogen atom, a C₁ -C₁₀ -alkyl group, a C₆ -C₁₀ -aryl group, a C₂ -C₁₀ -alkenyl group, a C₇ -C₄₀ -arylalkyl group, a C₈ -C₄₀ -arylalkenyl group or a C₇ -C₄₀ -alkylaryl group or R^(8') and R^(9'), or R^(8') and R^(10') each form a ring together with the atoms linking them, M' denotes tin and q denotes 1, 2 or 3, R⁶ and R⁷ are identical or different and denote a ═CR⁸ R⁹ group, in which R⁸ and R⁹ have the abovementioned meaning, m and n are identical or different and denote zero, 1 or 2, where m+n is zero, 1 or
 2. 2. The compound as claimed in claim 1, wherein said metallocene is a rac-bisindenyl(arylalkylsilyl)hafnium dichloride.
 3. The compound as claimed in claim 1, wherein R³ and R⁴ are identical.
 4. The compound as claimed in claim 1, wherein R³ and R⁴ are identical or different and are indenyl, tetrahydroindenyl or cyclopentadienyl radicals.
 5. The compound as claimed in claim 3, wherein R³ and R⁴ are indenyl radicals.
 6. The compound as claimed in claim 1, wherein R³ and R⁴ are indenyl radicals.
 7. The compound as claimed in claim 6, wherein R⁵ is ═PR⁸.
 8. The compound as claimed in claim 1, wherein R⁵ is ═PR⁸.
 9. The compound as claimed in claim 1, wherein R⁵ is ##STR14## --Sn--, --O--, or ═P(O)R^(8'), and wherein M' is tin.
 10. The compound as claimed in claim 1, wherein R⁵ is ##STR15## --Sn-- or --O--, and wherein M' is tin.
 11. The compound as claimed in claim 1, wherein R⁵ is arylalkylsilyl.
 12. The compound as claimed in claim 1, wherein said metallocene is rac-bisindenyl (phenylmethylsilyl)hafnium dichloride.
 13. The compound as claimed in claim 1, wherein R⁵ contains one of the formulas containing R⁸ and R⁸ is a C₆ -C₁₀ -aryl group, a C₇ -C₄₀ -arylalkyl group, a C₈ -C₄₀ -arylalkenyl group or a C₇ -C₄₀ -alkylaryl group.
 14. The compound as claimed in claim 13, wherein R⁸ is a C₆ -C₁₀ -aryl group.
 15. The compound as claimed in claim 14, wherein R⁸ is a C₆ -aryl group.
 16. The compound as claimed in claim 1, wherein R⁵ contains one of the formulas containing R^(8') and R^(8') is a C₆ -C₁₀ -aryl group, a C₇ -C₄₀ -arylalkyl group, a C₈ -C₄₀ -arylalkenyl group or a C₇ -C₄₀ -alkylaryl group.
 17. The compound as claimed in claim 16, wherein R^(8') is a C₆ -C₁₀ -aryl group.
 18. The compound as claimed in claim 17, wherein R^(8') is a C₆ -aryl group. 